maybe a boss round? Large necromacer like robot with different spells and it summons fast weaponless zombie-like robots that regenerate. Maybe the necromacer can only kill you with a spell that requires you to stay still and the zombie's goal is to kick you to the ground?

It would be cool if the necromancer guy would use a spell on the chopped up robots (but one at a time for balance I guess), and when he does it does an animation similar to that garbage pickup one, and the pieces float up and get put back in place! That would be bad ass to say the least.